A 36dB Gain Range, 0.5dB Gain Step Variable Gain Amplifier with 10 to 25MHz Bandwidth Third-Order Filter for Portable Ultrasound Systems

This paper describes a variable gain amplifier(VGA) and an anti-alias filter(AAF) used for low-power ultrasound imaging applications. The VGA has a digitally programmable gain from -12 dB to 24dB in 0.5dB steps for time gain compensation(TGC). The opamp used in the VGA consists of a two-stage opamp with an added pre-amplifier stage which increases the loop gain for signal frequencies without altering the unity-gain bandwidth. Improved loop gain results in lower harmonic distortion without a significant overhead in power consumption. The VGA has a first-order roll-off whose bandwidth is invariant with the gain setting. The AAF is a second-order Rauch filter which, together with the first-order roll-off in the VGA, implements a third-order Butterworth filter whose bandwidth is programmable from 10 to 25MHz in 5MHz steps. The VGA and the AAF, with a low noise amplifier(LNA) preceding them, are used in an ultrasound receive channel analog front-end(AFE) fabricated in a 180nm process. The power consumption and area are 3.6mW and 0.32mm2. The second and third harmonic distortion are -64 dBc and -56 dBc across VGA gain settings.